In the past, there are an electrophotographic MFP, a laser printer, and the like that record and form an image using a single laser element that irradiates one laser beam or using a plurality of the laser elements. However, recently, the electrophotographic MFP and the laser printer are mounted with a laser array instead of the laser element. In the laser array, plural laser elements are arranged at equal intervals. The electrophotographic MFP and the laser printer employing the laser array can record plural lines of an image in one scan.
In the past, in general, the resolution of an image forming apparatus is equal to or lower than 600 dpi. However, according to the technical improvement in recent years, image forming apparatuses having high resolution such as 1200 dpi and 2400 dpi are also put to practical use.
According to the progress of the increase in resolution, a new problem becomes obvious. Specifically, because of the progress of the increase in resolution, when an electrostatic latent image is formed, spots of beams irradiated on a photoconductive member overlap one another. If timing of one overlapping spot is different from timing of another, irregularity occurs in potential after exposure of the photoconductive member. This leads to image density unevenness.
This phenomenon is called reciprocity failure (JP-A-2004-20911). As techniques for reducing the influence of density unevenness due to the reciprocity failure, there are techniques explained below.
In a technique disclosed in JP-A-2004-77714, skip scanning is performed by using plural beams.
Therefore, time of scanning intervals for arbitrary scanning lines can be set to be equal to or larger than one main scanning time. As a result, an image quality defect due to the reciprocity failure is reduced.
In a technique disclosed in JP-A-2008-12806, it is determined whether overlapping writings are present in predetermined images adjacent to each other in a sub-scanning direction. Writing timing is changed according to presence or absence of the overlapping writings.